Abstract. Measurements of CO2, CO, N2O and CH4 mole fractions, O2 / N2 ratios and the stable isotopes 13C and 18O in CO2 and CO have been performed in air samples from the Islisberg highway tunnel (Switzerland). The molar CO : CO2 ratios, with an average of (4.15 ± 0.34) ppb:ppm, are lower than reported in previous studies, pointing to a reduction in CO emissions from traffic. The 13C in CO2 reflects the isotopic composition of the fuel. 18O in CO2 is slightly depleted compared to the 18O in atmospheric O2, and shows significant variability. In contrast, the δ13C values of CO show that significant fractionation takes place during CO destruction in the catalytic converter. 13C in CO is enriched by 3‰ compared to the 13C in the fuel burnt, while the 18O content is similar to that of atmospheric O2. We compute a fractionation constant of (−2.7 ± 0.7)‰ for 13C during CO destruction. The N2O : CO2 average ratio of (1.8 ± 0.2) × 10−2 ppb:ppm is significantly lower than in past studies, showing a reduction in N2O emissions likely related to improvements in the catalytic converter technology. We also observed small CH4 emissions, with an average CH4 : CO2 ratio of (4.6 ± 0.2) × 10−2 ppb:ppm. The O2 : CO2 ratios of (−1.47 ± 0.01) ppm:ppm are very close to the expected, theoretically calculated values of O2 depletion per CO2 enhancement.